Transcript CAM Photosynthesis - Mechanism

Undergraduate level Notes
Overview
 Temporal separation of C4 and C3 pathways of
photosynthesis: fixation by PEPC largely during the night
accumulates (usually) malate; decarboxylated largely
during the day for fixation by RuBisCO.
 Phasic
pattern of stomatal opening and closing, and
enzyme activity, facilitates the above.
 Titratable acidity can be used to quantify CAM activity.
Biochemistry
 Much like C4 (see resource): CO2, converted to HCO3- by
carbonic anhydrases, is initially incorporated into oxaloacetate
(OAA), along with phosphoenolpyruvate (PEP) from the
chloroplast, catalysed by PEP carboxylase (PEPC).
 OAA  malate (4C), by malate dehydrogenase, a reduction
step in which NADH  NAD+.
 The protonated form of malate, malic acid, is actively
accumulated in the vacuole, during the night, reducing the
vacuolar pH.
Biochemistry
 During the day, the malic acid diffuses back into the
cytosol.
 Malate is then decarboxylated in the chloroplast, yielding
CO2 for fixation by RuBisCO in the CBB cycle, and a 3C
compound.
 It is thought that it is the increasing internal CO2
concentration that causes the stomata to close.
CAM Phases
 It is a common misconception that the two sets of processes
outlined in the above two slides switch in their entirety
between day and night.
 However, the reality is more complex, and elements of each
process cycle differentially.
 Crucially, there is no dramatic shift from “night processes” to
“day processes” – elements of the processes shift gradually
between day and night.
CAM Phases
 It is possible to identify 4 phases of CAM
 Phases I and III correspond respectively to the night
processes and day processes
 2 transient phases (II and IV) may allow additional CO2
fixation under certain environmental conditions.
CAM Phases
CAM Phases
 Phase I (night): stomata open; fixation by PEPC; malic acid
accumulation.
 (Phase II [early morning]: stomata still open; switch from PEPC 
RuBisCO accompanied by burst of CO2 fixation; beginning of
deacidification).
 Phase III (day): stomata closed; deacidification as malate
decarboxylated; net fixation by RuBisCO; build up of carbohydrates.
 (Phase IV [late afternoon]: if plant well watered, stomata may open
before nightfall allowing direct C3 photosynthesis by RuBisCO to take
place).
PEPC Regulation
 How can plants ensure that the correct processes take
place at the optimum time? Answer: by circadian
(endogenous daily rhythmic) control of the enzymes
involved, in this case PEPC.
 De novo synthesis of a specific PEPC kinase at night
(under circadian control) allows PEPC to be
phosphorylated to its active form – the dephosphorylated
“day” form is highly sensitive to inhibition by malic acid,
and is therefore inactive.
Variation on the Pathway
 Much like C4, the exact details of the CAM biochemistry varies.
 Malic acid is accumulated in most if not all CAM plants, however
some species additionally accumulate citric acid (e.g. Some
strangling figs, and pineapple).
 As in C4, the enzyme responsible for the decarboxylation step, and
the product of this step varies between species – see the C4
resource for examples of this variation.
 A further variation present in CAM plants is the extent to which they
employ CAM (see next  )
Inducible CAM
 Unlike C4, which is usually associated with Kranz anatomy and
is therefore either present or absent, CAM can either be
constitutively employed (“obligate” CAM plants) or inducible
(“facultative” CAM plants).
 Inducible CAM is often present in plants whose environment
cycles between, e.g., drought (when CAM can help conserve
water) and water abundance, in which C3 photosynthesis is
sufficient and more cost effective. A prime example is the
“iceplant”: Mesembryanthemum crystallinum, which switches
from C3 to CAM photosynthesis under water or salt stress.
Calculating CO2 Fixation
 The accumulation and decarboxylation of acid in a pattern related to
CO2 fixation provides a convenient method by which to quantify
such aspects (and more) of the CAM cycle.
 By collecting tissue samples at intervals across a 24hr period and
titrating the extract to neutrality, it is possible to calculate the
concentration of H+ in the tissue (i.e. one can calculate the
“titratable acidity”).
 Given the direct stoichiometric relationship between CO2 : H+:
malate of 1 : 2 : 1, the titratable acidity can easily be used to
determine the levels of CO2 fixation by PEPC that are occurring.
Summary
 CAM is a temporal separation of C3 and C4 photosynthetic
processes.
 Variable phases are regulated daily on both a circadian and
environmental basis.
 CAM pathways are highly variable between species and may
be constitutively present or inducible.
 Dawn-dusk titratable acidity is a useful measure of CO2 fixation
by CAM plants.